Plasma torches are a source of high temperature, high velocity gas, and are currently used in many applications, including plasma spraying, powder manufacture, materials processing, spray forming, cutting or heat processing. Plasma spraying is used to spray a coating of feedstock onto a metal, ceramic or other substrate material, in order to cause the feedstock to become adhered to the substrate as a thin coating on the substrate. A plasma stream is generated by an arc formed between a cathode and an anode in spaced apart relationship within a chamber. The forming of the are and the consequent generation of the plasma stream are usually done in inert gases, such as argon, to avoid corrosion and other deterioration of the cathode electrode. Secondary gases, such as hydrogen, nitrogen or helium, may be added to the plasma gas in order to increase plasma heat content and thermal conductivity.
Feedstock is injected into the plasma stream causing the feedstock to melt and become propelled by the plasma stream out of the plasma torch onto the substrate material. Existing plasma torches generally provide for feedstock injection into the plasma stream in a direction radial or perpendicular to that stream. The feedstock passage opens perpendicularly into the plasma chamber and feedstock is carded laterally into the stream by means of a carrier gas. Preferably, the feedstock injection takes place downstream from the are forming chamber, generally in the proximity of the plasma torch exit nozzle.
The radial injection of feedstock suffers from several disadvantages. The main disadvantage is the effect of particle segregation of the feedstock between the point of injection into the plasma stream and the deposit surface. This results in non-uniform particle temperature and velocity distribution and divergent particle trajectory which has a negative effect on coating properties and deposition efficiency. These deleterious effects can be avoided when feedstock is fed centrally, or axially, into the axis of a plasma stream in the direction of stream flow, resulting in less divergent particle trajectories and velocities and more uniform heat transfer.
Much of the prior art directed to axial injection of feedstock into a plasma stream provides for multiple systems incorporating a plurality of plasma generators disposed symmetrically about a common axis. Feedstock is injected into the resultant combined stream at or near the area where the streams are brought together. A plurality of independent plasma streams are formed and brought together along the common axis. Examples are found in U.S. Pat. No. 4,982,067 of Marantz, et al and U.S. Pat. No. 5,008,511 of Ross.
The Marantz, et al patent provides a plurality of generated plasma arcs which coalesce toward a common anode electrode. A nozzle injects feedstock and a secondary gas axially into the coalesced plasma stream. The feedstock is injected prior to the anodic electrode attachment which may result in particle overheating and premature melting through direct interaction with a highly ionized plasma are. This is undesirable and can result in deposit of feedstock particles on components of the plasma torch, rather than ejection and deposit on the substrate surface.
The Ross patent also provides a plurality of plasma generators arranged symmetrically about a common axis. A plurality of independent plasma streams are generated and directed into a common region of convergence downstream of the anode. Feedstock is fed axially into the resultant coalescent plasma stream in the region of convergence.
These kinds of axial feed plasma torches all rely on several independent plasma generating sources which results in a high degree of complexity and cost of manufacture and operation. As well, it is common for arcs which are generated by multiple electrode systems to be of differing magnitude forming plasma streams of differing velocity and intensity. When these irregular plasma streams are brought together, difficulties in achieving an axially uniform coalescence of the separate streams into one stream may occur. As well, an irregular united stream results in increased radial drag which causes feedstock particulates to deviate from an axial direction and deposit on components of the plasma torch or travel outside the main plasma stream, thus receiving insufficient heat.